For over 90 years, annuli between oil well casings and surrounding boreholes have been cemented to prevent vertical communication of formation fluids outside of the casing. This has, in the past, been accomplished by placing the casing in the well bore, and then circulating a cement slurry down the casing and up the annulus around the casing then allowing the cement to set in the annulus. Circulation of the cement slurry into place this way does not always effectively displace drilling fluids. Drilling fluids that are not displaced by cement slurries create voids in the set cement. These voids can result in channels for undesirable vertical communication of formation fluids.
Voids often result from the casing not being effectively centralized. When the casing is close to the borehole wall along one side of the casing the cement slurry will then tend to flow up the wider portion of the annulus and bypass portions of the annulus where the casing is near the wellbore wall. Drilling fluids are also difficult to displace from washed-out or otherwise enlarged portions of the wellbore. Such voids in casing cement are not only difficult to avoid, but are very difficult to correct after the cement has hardened.
Difficulties in displacing drilling fluids are compounded by incompatibility between most drilling fluids and Portland type cement slurries.
Displacement of drilling fluids in extended reach drilling is particularly difficult not only due to typically poor centralization, but a tendency for slurries having a density different from the drilling fluid to form a layer over or under the drilling fluid. Slimhole drilling results in additional difficulties in removing drilling fluids because of the more narrow annulus this technique creates.
It is therefore an object of the present invention to provide a method for cementing a casing into a wellbore wherein voids in the cement due to poorly centralized casings are eliminated. It is a further object to provide such a method wherein washouts and other enlarged portions of the borehole are filled with cement.